1. Field of the Invention
The present invention relates to a solid state imaging device utilizing a charge transfer device.
2. Technical Background
A first example of a block diagram of a conventional solid state imaging circuit is shown in FIG. 5. FIG. 5 shows a part of the circuitry of a conventional solid state imaging apparatus based on the so-called interline transfer method. This type of apparatus comprises a plurality of vertical charge transfer sections 501 each having a plurality of line charge transfer elements; a plurality of photo-electric conversion elements 502 arrayed in rows and columns, each column being disposed adjacent to one side of the associated vertical charge transfer section 501; a horizontal charge transfer section 503 electrically coupled with a terminal end of each vertical charge transfer section 501; and an output section 504 disposed on one end of the horizontal charge transfer section 503.
FIG. 6 shows an enlarged view of the device structure of a portion 505 as shown in FIG. 5. This portion indicates respective parts of the vertical charge transfer section 501 and the horizontal charge transfer section 503. In FIG. 6, the region bounded by the dotted line represents a channel region which comprises a channel region 601 inclusive of the vertical charge transfer section, and a channel region 605 inclusive of the horizontal charge transfer section. The channel region 601 of the vertical charge transfer section is fabricated with first vertical charge transfer electrodes 602, 604 which are overlaid with a second vertical charge transfer electrode 603 and a final vertical charge transfer electrode 604 functions as the transfer electrode to transfer signal charges from the vertical charge transfer section 501 to the horizontal charge transfer section 503.
The channel region 605 of the horizontal charge transfer section is fabricated with first horizontal charge transfer electrodes 606, 607 which are overlaid with second horizontal charge transfer electrodes 608, 609. In general, these electrodes are fabricated from double layers of polysilicon, and the first horizontal electrodes 606, 607 and the first vertical electrodes 602, 604 are fabricated on the first polysilicon layer; and the second horizontal electrodes 608, 609 and the second vertical electrode 603 are fabricated on the second polysilicon layer. This unit device pattern, constituted by the electrodes 606, 607, 608 and 609, is repeated in the horizontal direction. The channel region 605 of the horizontal charge transfer section comprises: a first region 611, of the channel region of the horizontal charge transfer section, serving as the charge storage region; and a second region 612, of the channel region 605 of the horizontal charge transfer section, serving as the charge barrier region.
The first horizontal charge transfer electrode 606 operates in a pair with the second horizontal charge transfer electrode 608, and the first horizontal electrode 607 operates in a pair with the second horizontal charge transfer electrode 609, to respectively constitute a pair of electrodes across the two polysilicon layers. Each pair of electrodes receives a clock pulse signal of its own having a 180 degree opposing phase relationship to the other clock pulse signal, such as .phi.H.sub.2 and .phi.H.sub.2 shown in FIG. 7. The first horizontal electrode 606 and the second horizontal electrode 608 receive the same pulse signal, .phi.H.sub.1 for example. The first horizontal electrode 606 is fabricated on the first region 611 serving as the charge storage region for the horizontal charge transfer section, and the second horizontal electrode 608 is fabricated on the second region 612 serving as the charge barrier section for the vertical charge transfer section; therefore, even under the same applied pulse signal, a potential difference .phi. is generated between the channel regions 611, 612. This potential difference .phi. provides the directionality in the charge transfer process. Furthermore, the terminal end of the channel region 601 of the horizontal charge transfer section (shown at far left in FIG. 6) comprises: a first region 613 (of the channel region of the vertical charge transfer section); and a second region 614 (of the terminal end of the channel region of the vertical charge transfer section), in which the first region 613 is overlaid with a part of the horizontal charge transfer electrode 606, and the second region 614 is overlaid with a part of the second horizontal charge transfer electrode 608. The second horizontal charge transfer electrode 608 in the regions 613, 614 is shaped orthogonal so as to cover over the spaces between the first vertical charge transfer electrode 604 and a part of the first horizontal charge transfer electrode 606. This orthogonal part overlays the second region 614 disposed at the terminal end of the vertical charge transfer section.
The second region 614 (of the terminal end of the channel region of the vertical charge transfer section) serves as the charge barrier region formed at the coupled region between the vertical charge transfer section 501 and the horizontal charge transfer section 503. The fact that the second region 614 (which serves as the charge barrier region) is driven by the the second horizontal charge transfer electrode 608 assures that the pulse signals applied on the first horizontal charge transfer electrode 606 and on the second horizontal charge transfer electrode 608 are in phase with the electrical potential of the barrier region.
Therefore, when the voltage applied on the horizontal charge transfer electrodes 606, 608 changes from a high level to a low level, in other words, when the charges stored in the first horizontal charge transfer electrode 606 are transferred in the horizontal direction, the voltage potential at the charge barrier region also becomes shallow, thus preventing the charges (to be transferred) from reversing back to the vertical charge transfer section 501 by going over the charge barrier region. This technology is disclosed in detail in a Japanese Laid-Open Patent Application No. Sho58-125969.
FIG. 8 is a schematic representation of a cross section of the coupling region along a line I--I shown in FIG. 6. The first and second vertical charge transfer electrodes 604, 603; the first and second horizontal charge transfer electrodes 606, 608; are fabricated on the top surface of a semiconductor substrate 801 (p type) with an intervening insulation film 802. Formed in the substrate 801 under the first and second vertical charge transfer electrodes 604 and 603 and the first horizontal charge transfer electrode 606 are buried channel layers 803-1 and 803-2 having an opposite conductivity type (n type) to the substrate 801 (p type). The layer 803-2 serves as charge storage regions 611 and 613. Below the second horizontal charge transfer electrode 608, a buried channel layer 804 of an opposite conductivity type (n.sup.- type) is formed, and serves as the charge barrier region (612/614). The electrical potential of the second regions 612, 614 (charge barrier region) is made smaller than that of the first regions 611, 613 (charge storage region). Outside regions of the channel regions are represented as a channel stopper region 805 (p.sup.+ type) of the same conductivity type as the semiconductor substrate 801.
The operation of the solid state imaging apparatus of the configuration presented above will be described with reference to FIG. 5.
Signal charges, accumulated in relation to the energy of the light inputted into the photo-electric converter elements 502, are read out by the vertical charge transfer sections 501 in accordance with the frame period or the field period of the imaging signal. Subsequently, the signal charges are transferred down successively inparallel within the group of the relevant vertical charge transfer sections 501. The signal charges thus transferred to the terminal end of the group of the vertical charge transfer sections 501 are transferred in parallel to the horizontal charge transfer section 503 in accordance with the horizontal transfer timing. The signal charges transferred to the horizontal charge transfer sections 503 are transferred successively in the horizontal direction while the next period of signal charges are being transferred from the group of vertical charge transfer sections 501. The signal charges are then outputted from the signal output section 504 as imaging signals.
The details of the transfer process of signal charges from the vertical charge transfer sections 501 to the horizontal charge transfer sections 503 will be explained with reference to FIGS. 9A and 9B. Note that FIG. 9A shows a cross section of the coupling regions of the signal charge transfer device and FIG. 9B shows a schematic illustration of the distribution of the relevant electrical potentials. The signal charges are accumulated in the terminal electrode during the ON-period (shown in the potential diagram by the dotted line) of the vertical charge transfer section, and the accumulated charges are transferred via the second region 614 and first region 613 to the first region 611 for charge storage when the terminal electrode enters the OFF-period (shown in the potential diagram by the solid line). In this case, even though both regions 611 and 613 are channel regions which are overlaid with the horizontal charge transfer electrode 606, their electrical potentials, .phi.H and .phi.V.sub.2, for these regions 613, 614 are different, as shown by the steps in the potential diagram. The difference in the potential .phi. is caused by the fact that the first region 613 is disposed in the channel regions of the vertical charge transfer section, and its channel width W.sub.V2 is narrower than the channel width W.sub.H Of the horizontal charge transfer section. The channel width shown by W.sub.H in FIG. 9, for the horizontal charge transfer section, is the major channel width for the first region 611. When the channel width becomes narrow, the electrical field leaks to both sides of the channel, and the wasted area becomes no longer negligible, and therefore, as the channel width becomes narrow, the electrical potential associated with the narrow channel region becomes low. This is the so-called narrow channel effect.
An example of the electrical potential dependence of the channel width is shown in FIG. 10, which shows that the electrical potentials are different for different channel widths. For this reason, the stepwise potential distribution shown in FIG. 9B is produced.
In the conventional solid state imaging apparatus, the generation of black vertical lines in the process of charge transfer from the vertical charge transfer section 501 to the horizontal charge transfer section 503 is prevented by using the narrow channel effect described above, by means of a fringe electrical field having such stepwise distribution of potential from the vertical section 501 to the horizontal section 503, so as to shorten the transfer time for the signal charges to transfer from the vertical charge transfer section 501 to the horizontal charge transfer section 503.
However, with the trend towards densification of pixels while making the apparatus compact, there has also been a trend towards coupling bonding with shallow buried channels, so as to improve the charge transfer efficiency per unit area of the vertical charge transfer section. In such technology, there is a problem that the generation of vertical black lines increases due to the fact that the strength of the fringe electrical field of shallow buried channels is insufficient, causing the charge transfer time, i.e. efficiency, to increase longer from the vertical charge transfer section to the horizontal charge transfer section. The generation of vertical black lines thereby become a severe problem.
A remedy procedure for this problem is disclosed in a Japanese Patent Laid-Open Application No. Sho63-14467, in which the channel spacing W of the vertical charge transfer electrode 604 is widened to allow an increase in the fringe electrical field strength. However, this technique applied to the conventional technology described above results in little increase in the channel width W.sub.V ' as shown in FIG. 11, and little improvement in performance. On the other hand, if the channel width is increased to W.sub.V " as shown in FIG. 12, it naturally occurs that the spacing of the first horizontal charge transfer electrodes 606 is increased from L to L'. To retain the channel spacing between the photo-electric converters at W (i.e., without increasing the width to W'), the electrode widths for the other electrodes must be narrowed. However, such an approach would lead to an unbalance in the electrode configuration in the horizontal charge transfer section, and leads to significant design and performance problems in the charge transfer capacity and speed.